1. Field of the Invention
This invention relates to methods of making metallic magnetic cobalt-phosphorus alloy particles by controlled chemical reduction under the influence of a D.C. magnetic field, and to the use of such particles in magnetic recording media and magnetic recording systems. It does not relate to the treatment of magnetic particles in a magnetic field after the particles have come into existence.
2. Description of the Prior Art
Both the production of cobalt-phosphorus particles by chemical reduction and the use of a magnetic field during the production of magnetic particles and magnetic films are known in the prior art. However, the problem of defining cobalt-phosphorus magnetic particles produced by chemical reduction which are suitable for high density recording in terms of the figure of merit herein designated as W/H.sub.c, the importance and interpretation of W/H.sub.c and the effect of a specific type (D.C.), and magnitude (at least about 200 gauss) of magnetic field on the W/H.sub.c of cobalt-phosphorus particles during their production have not previously been appreciated. For example, prior art teachings of the use of a magnetic field during the production of magnetic material have often indicated that the use of either an A.C. or a D.C. magnetic field was interchangeable, have designated neither A.C. nor D.C., or have failed to designate the required minimum magnitude of the field.
It has long been theorized that metallic magnetic materials have characteristics which should make them more suitable for use in magnetic recording media than lower specific moment materials, such as iron oxide, for example. However, there is currently no large volume or general purpose recording media which is based upon the use of particulate magnetic metal.
Historically, the very first magnetic recording media developed were in the form of continuous metallic magnetic wires or bands. Initially, the use of solid magnetic metal recording media was quite satisfactory. Magnetic metals exhibited good properties of saturation magnetization and were easy to manufacture. However, they eventually fell out of favor due to their physical characteristics. For example, magnetic wire under tension tended to twist during both recording and playback, and as a result exhibited non-uniform signal output. Additionally, metallic magnetic wires and bands were bulky to store, heavy, and possessed relatively high inertia. This latter characteristic limited their utility in high speed stop/start/reverse and high density recording systems.
Subsequently, metallic magnetic wires and bands were replaced, primarily by light weight media formed of particulate magnetic material in a polymeric binder coated onto a substrate, usually in the form of a flexible tape. Such particulate magnetic tape had the advantage of being lower in bulk, weight, and inertia than solid metallic magnetic media. Additionally, the use of a tape structure, as opposed to wire avoided any tendency for the media to twist. Where magnetic iron oxide was the magnetic particle of choice, the media exhibited recording characteristics substantially as good as the solid magnetic metallic media it replaced, with the exception of saturation magnetization. High saturation magnetization is desirable, as it allows the recording and playback of a large or strong signal, and is thus especially desirable for high density recording.
Attempts were made to improve the saturation magnetization of low bulk, low weight, flexible media by utilizing thin magnetic metallic films deposited on a substrate by various coating techniques. However, such "plated" tapes tended to be susceptible to corrosion and wear to an extent that made them impractical, especially for high speed, high density recording system use.
Some efforts to provide a superior high density magnetic recording media attempted to substitute metallic magnetic particles for magnetic iron oxide. It was assumed, quite reasonably, that metallic magnetic particles having magnetic and physical characteristics equal to or better than iron oxide would provide particulate magnetic recording media superior to media based upon magnetic iron oxide. Despite the theoretical utility of metallic magnetic particles, they did not normally provide a signal superior to the signal from comparable iron oxide media, at the same volume or weight loadings as the iron oxide particles. Additionally, heretofore, particulate magnetic recording media utilizing metallic magnetic particles have exhibited disappointingly poor resolution, erasability and saturability.
We thus find, in the prior art, the anomalous disagreement between theory and practice, which has led away from the utilization of metallic magnetic particles in particulate media.
As was indicated above, many techniques of making metallic magnetic particles, including the preparation of cobalt-phosphorus particles by chemical reduction, are known in the art. Additionally, many prior art techniques for making magnetic particles have utilized some form of magnetic field during the actual preparation of the particles. However, heretofore, the prior art has not provided a suitable figure of merit for characterization of cobalt-phosphorus or other metallic magnetic particles for use in recording media and then utilized that figure of merit to recognize the improvements which could be obtained in metallic magnetic cobalt-phosphorus particles produced by chemical reduction by the application of a D.C. magnetic field of the proper magnitude during particle preparation. Without the recognition of the problem, the preparation of metallic magnetic particles by chemical reduction would remain a matter of chance, subject to inexplicable and non-reproducible successes and failures.